In many communications systems, it is desirable for several remote terminals (transmitters) to communicate data over a shared communications medium to a central node (receiver). For example, the shared medium might be a portion of radio spectrum. FIG. 1 shows a generic setting where k transmitters 2 are communicating via a shared medium 3 with a common receiver 4.
One challenge in such scenarios is how to efficiently share the medium between the different transmitters. In situations where centralised control is possible, the transmitters can agree to access the channel using some orthogonal access scheme, such as time division multiple access or frequency division multiple access. Other methods include code division multiple access and orthogonal frequency division multiple access. Such schemes require very tight time and frequency synchronisation between all the transmitters in order to maintain orthogonality between the user's signals. This can be demanding, or expensive to achieve.
Furthermore, in many situations of interest, the communications medium may be dispersive in time or frequency (or both). One example of such a channel is the multipath mobile radio channel, where relative motion of transmitters and receivers can induce Doppler offsets. Reflections of radio signals from the environment also results in the superposition of many copies of the transmitted waveform, each with its own time delay, attenuation and phase offset. Another example of a channel which can introduce significant time and frequency offsets is in the context of communication with a low earth orbit satellite. In this situation, the time-of-flight to the satellite can vary significantly depending on the relative position of the satellite with respect to the transmitter. Furthermore, the high speed of the satellite (as seen from a fixed point on the ground) induces large Doppler offsets.
In such situations, the satellite channel can impose (potentially time-varying) time, frequency and phase offsets to each of the transmitted signals. In some situations the frequency offset may even be time-varying, according to some frequency rate (measured in radians/second2 or Hz/second). These conditions can also occur in some terrestrial cases, such as when the receiver is an airborne receiver, or transmitters are fast moving relative to the receiver (eg the transmitters are airborne) in which case the communications channel will be referred to as a satellite-like channel.
These offsets may not be known a-priori to the transmitters, which means that attempts to transmit using orthogonal access schemes can be defeated by the channel. Although the transmitted signals are orthogonal, they may be non-orthogonal when they arrive at the receiver. This causes multiple-access interference, whereby the signals from different transmitters mutually interfere at the receiver. This can seriously degrade the performance of the system.
One approach to address this problem is to estimate the relevant time, frequency and phase offsets at the receiver and use a feedback channel to provide these estimates to the corresponding transmitters so that they can pre-compensate for these effects. This approach not only requires a dedicated feedback channel, it must operate fast enough such that the channel parameters do not change too much from when they were estimated to when they are used.
Another approach is to use guard bands and guard intervals to provide sufficient time and frequency separation so that no matter what offsets are introduced by the channel, the signals from different, transmitters do not interfere. This approach is simple, but still requires sufficiently accurate synchronisation between the transmitters. Furthermore, it is wasteful of bandwidth, which may be a problem if a spectrally efficient system is required.
There is thus a need to provide an improved multiuser communications system that is adapted for use in cases where there are multiple transmitters transmitting to a receiver, wherein the multiple transmissions mutually interfere and are transmitted over a satellite or satellite-like channel to the multiuser receiver, or to at least provide a useful alternative to existing systems.